1. Field of the Invention
The present invention relates to the technical field of a driving circuit applied in a light emitting diode (LED) lamp, and more particularly to a purely resistive dimming circuit that supports a TRI-electrode AC switch (TRIAC) dimming, drives the LED to emit lights at different stages, and uses the impedance property of a thermistor to overcome the problems of the conventional driving circuit of an incandescent lamp that cannot be operated at a constant power and has a low dimming efficiency. Therefore, the purely resistive driving circuit is applicable in the LED lamp and substitutes the existing complicated circuit structure having an inductor, a control chip and/or a holding current, etc.
2. Description of the Related Art
In general, most control circuits of an LED lamp are switch components for controlling the phase, such as a silicon controlled rectifier (SCR) or a TRIAC for changing the voltage phase of an input power, and changing the magnitude of the driving voltage outputted from the control circuit by the method of switching the conduction angle of the voltage phase, so as to adjust the intensity of driving current passing into the LED light source and achieve a dimming effect. Although the dimming device of this sort has the advantages of simple control and convenient installation, yet the voltage waveform of the input power is distorted to cause problems such as low PF and increased voltage harmonics. According to the current/voltage (IN) characteristic curve of the LED, the LED is not a linear component; in other words, voltage and current are not in direct proportion. Therefore, the aforementioned dimming method may have an imprecise dimming effect due to the unequal variation of the driving voltage and the driving current, and thus causing a reactive power. In addition, if the operating voltage flowing into the control circuit is affected by the property of the TRIAC component and becomes too low, and the current passing through the TRIAC is lower than the required operating current, the TRIAC will switch its working status repeatedly, and the driving current will be non-continuous, and the LED will blink, so as to reduce the illumination quality.
With reference to FIG. 1 for the circuit diagram and waveform diagram of a conventional LED driving circuit having a holding current, the driving circuit 1 comprises a dimmer 10 being a TRIAC element, a bridge rectifier 11, a holding switch 12, a holding resistor (RH) 13 and a filter capacitor 14, wherein the bridge rectifier 11 is provided for receiving an external power through the dimmer 10 and rectifying the power into an input voltage (Vin) and an input current (Iin), and the filter capacitor 14 receives and converts the input current into a driving current (IL) to be supplied to the LED. In addition, the holding switch 12 is an N-type Metal Oxide Semiconductor Field-Effect Transistor (N-type MOSFET) having a drain coupled to the bridge rectifier 11 for receiving an input current, a source coupled to the holding resistor 13, and a gate for receiving a holding voltage (VH), so that the holding resistor 13 receives the holding voltage through the holding switch 12 to output a holding current (Ihold), and Ihold=(VH−VGS_N-MOSFET)/RH, and the holding current (Ihold is provided for maintaining the dimmer 10 at a stable operating status to improve the dimming precision). However, the installation of the holding switch 12 and the holding resistor 13 undoubtedly has a certain power loss and affects the overall circuit working efficiency, and thus causing a low energy utilization rate and failing to comply with the requirements of electric appliances.
In view of the aforementioned problems of the conventional dimming circuit, it is a main subject of the present invention to simplify the present driving circuit structure and supports TRIAC dimming and still maintains a constant driving current without requiring the components such as inductor, control chip and/or holding current, so as to achieve the effects of ensuring the illumination quality of the lamp and lowering the manufacturing cost.